Method for controlling stack-advancing in a reproduction apparatus

ABSTRACT

The present invention relates to a method for providing paper stack level calibration in a reproduction apparatus. According to various aspects of the invention, methods are provided for continuous feeding with a transition from one supply to another, and leaving a controlled number of sheets in the prior supply.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of pending U.S. patentapplication Ser. No 10/668,417, filed on Sep. 23, 2003, by Thomas K.Sciurba, et al., entitled “METHOD FOR CONTROLLING STACK-ADVANCING IN AREPRODUCTION APPARATUS” hereby incorporated by reference and hereinassigned to the Eastman Kodak Company.

FIELD OF THE INVENTION

The present invention relates to a method for providing paper stacklevel calibration in a reproduction apparatus.

BACKGROUND OF THE INVENTION

In typical reproduction devices, such as copiers or printers, forexample, information is reproduced on individual cut sheets of receivermaterial such as plain bond or transparencies. Receiver sheets of thevarious types are stored in stacks and respectively fed seriatim fromsuch stacks when copies are to be reproduced thereon. The sheet feederfor the reproduction devices should be able to handle a wide range ofsheet types and sizes reliably and without damage. Desirably, the sheetsare accurately fed individually from the sheet stack without misfeeds ormultifeeds.

Reproduction device sheet feeders are typically of two types, vacuumfeeders or friction feeders. An exemplary vacuum sheet feeder is shownin U.S. Pat. No. 5,344,133, issued Sep. 6, 1994, in the name of Jantschet al. In such an apparatus, a stack of sheets is stored in a supplyhopper. A sheet feed head assembly, including a plenum, a vacuum sourcein flow communication with the plenum, and a mechanism, such as a feedbelt associated with the plenum, transports a sheet acquired by vacuumin a sheet feeding direction away from the sheet supply stack.

Typically, in most vacuum sheet feeders, the sheet supply stack issupported to maintain the topmost sheet at the feed head assembly. Afirst positive several sheets in the supply stack to an elevationenabling the topmost sheet to be acquired by vacuum from the sheet feedhead assembly plenum. Additionally, a second positive air supplytypically directs a flow of air at an acquired sheet to assureseparation of any additional sheets adhering to such topmost sheet.

It is clear that the sheet stack should be maintained in a particularpositional relation with the sheet feed head assembly to assure desiredfeed from the stack. An exemplary control of a sheet stack is shown inU.S. Pat. No. 5,823,527, issued Oct. 20, 1998, in the name of Burlew etal. In such an apparatus, a sheet feeder is disclosed having a platformfor supporting a stack of sheets, a feed head assembly for feedingsheets seriatim from the top of a sheet supply stack on the platform, amechanism for moving the platform relative to the feed head assembly,and device for controlling operation of the platform moving mechanism.The control device can determine a selected parameter in response toexamination of sheet stack parameters, and consequently produce a signalcorresponding thereto. The speed of the platform moving mechanism isthen set based on the parameter signal.

Modem reproduction devices have more than one sheet feeder to storedifferent types of sheets. When running large print jobs without anystop page there is a need to switch over from one feeder to another.Normally the first stack is not run empty before switching over to thenext stack. It is preferred to leave the minimum number of sheetsnecessary to insure that the feed source will not run out prior toswitching. This maximizes the effective capacity of the supplies andminimizes the number of sheets that are likely to be exposed toundesirable environments for an extended period of time as a result ofbeing left behind. Normally, feeding is switched to another feed sourcewhen a paper low condition is signaled. This is typically determined bysensing that the platform has reached a certain position, either throughaction of a switch, or feedback from a platform travel monitor, such asan encoder, potentiometer or step count from a step motor. The actuationpoint for this paper low condition is selected to insure that asufficient number of receiver sheets is present to allow switching underall conditions. Due to the system architecture, the system tolerancesand differences in the receiver sheet thickness, this actuation point isselected conservatively. This results in an excessive number of sheetsremaining under most conditions.

The stack advancing is often performed with stepper motors. The heightposition of the stack is proportional to the number of steps a steppermotor is triggered. The paper supply controller needs data relating tothe displacement of the stack supporting platform relative to a downswitch for several reasons. The displacement data is used to determinethe paper low status as well as enabling the paper out check and otherfunctions. The paper low displacement is one parameter that determineshow many sheets are left behind in a supply hopper after a continuousmode swap, wherein paper supplies are switched and filled alternately inorder to provide continuous stream of sheets to the marking engine. Asmentioned before, the displacement can be measured in terms of steppermotor steps applied. The mechanical tolerances in the stack advancingmechanism are such that no nominal value for each of these displacementswould give an acceptable performance for all supplies of thereproduction apparatus. Although it is possible to manually calibratethe total possible displacement of an elevator, it is inconvenient tomanually calibrate for paper thickness.

The embodiments described herein allow for more effectively controllingthe level of a sheet stack and the switching over to the next stack.

SUMMARY OF THE INVENTION

According to various aspects of the invention, methods are provided forcontinuous feeding with a transition from one supply to another, andleaving a controlled number of sheets in the prior supply.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of an exemplary receiver sheet supplyand feeding apparatus.

FIG. 2 is a top plan view of the receiver sheet supply and feedingapparatus of FIG. 1, with portions removed or broken away to facilitateviewing.

FIG. 3 is a side elevational view of a cross-section of the receiversheet supply and feeding apparatus taken along lines 3-3 of FIG. 2,particularly showing the platform elevating mechanism.

FIG. 4 is an end view, on an enlarged scale and with portions removed,of a portion of the receiver sheet supply and feeding apparatus,particularly showing the feed head assembly thereof, taken along thelines 4-4 of FIG. 3.

FIG. 5 is a schematic illustration of an exemplary reproduction devicewith two feeding apparatuses.

FIGS. 6-9 present a schematic illustrations of a different stackadvancing scenes according to further aspects of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Addressing the problems with paper feeder supplies in reproductiondevices described above, the present embodiments provide effectivecontrol of a paper stack in a reproduction apparatus with the capabilityof increasing the effective receiver sheet capacity.

According to an aspect of the present invention, the control ofstack-advancing may be characterized by an elevator step calibrationmanagement system whereby each supply will calibrate itself for both thetotal possible displacement and the paper low displacement of a stacksupporting platform. The calibration occurs in a fashion that is bothcontinuous and independent from the user. The calibration procedurecould be performed every time a stack has been renewed or the sheetattributes were changed.

According to another aspect of the invention, the number of elevatorsteps counted during the calibration procedure could be checked withpreset values to eliminate malfunctions in the stack advancing controland devices.

According to another aspect of the invention the data derived from thecalibration procedure could be used to control the switching over to thenext stack and to calculate the limits for declaring elevator movementproblems.

The present invention provides a number of advantages and applicationsas will be readily apparent to those skilled in the art. Utilizing thedisclosed methods, the present invention allows increased effectivereceiver capacity without increasing the risk of running out of paperwhile feeding sheets and switching over to another stack.

The present embodiments described herein, provide the ability to moreeffectively control a paper stack in a reproduction device. The systemand method have been implemented in a reproduction device utilizing atop feed vacuum feeder. However, it should be understood that thepresent embodiments can be implemented in a reproduction device thatutilizes other types of feeders, including variations of the vacuumfeeder or a friction feeder. Thus, the exemplary embodiments disclose asystem and method that can be utilized to increase the efficiency forany type of reproduction machine.

FIG. 1 is a side elevational view of an exemplary receiver sheet supplyand feeding apparatus according to one aspect of the invention. Thereceiver sheet supply and feeding apparatus 10 generally includes anopen hopper 12 and an elevating platform 14 for supporting a stack ofsheets. The sheet stack (not shown in FIG. 1) supported on the platform14 contains individual sheets suitable, for example, for serving asreceiver sheets for having reproductions formed thereon in a copier orprinter device. Sheets for receiving reproductions may be selected froma wide variety of materials and sizes, which altogether define the sheetattributes. For example, the sheets may be of a weight in the range of49 grams per square meter (“gsm”) to 300 gsm index, and a size in therange of 8·times·10 inches to 14·times·18 inches, or larger, or smaller,depending upon the application.

The sheet stack supporting platform 14 is supported within the hopper 12for substantially vertical elevational movement by a lifting mechanism(“L”). Preferably, the lifting mechanism L serves to raise the platform14 to an elevation for maintaining the topmost sheet in the stack at apredetermined level during operation of the receiver sheet supply andfeeding apparatus 10, and to lower the platform to permit adding sheetsthereto. The lifting mechanism L may include a motor (“M·sub·1”),attached to the outside of the upstanding front wall of the hopper 12.Preferably, the motor M·sub·1 rotates a gear set 16 mounted on a shaft18 extending from the upstanding rear wall of the hopper 12. A pair ofsprocket mounted lifting chains 20 are respectively interconnected bygears with the shaft 18 to be moved about a closed loop path when theshaft 18 is rotated by the motor M·sub·1. As shown in FIG. 1, the sheetstack supporting platform 14 is shown in its lowest position in phantom.This most bottom position of the platform 14 is detected with a downswitch 21.

FIG. 2 is a top plan view of the receiver sheet supply and feedingapparatus 10 of FIG. 1, with portions removed or broken away tofacilitate viewing of a sheet feed head assembly 30. The sheet feed headassembly 30 is generally located in association with the hopper 12, soas to extend over a portion of the platform 14 in spaced relation to asheet stack 50 supported thereon. The sheet feed head assembly 30includes a ported plenum 32 connected to a vacuum source V, and an airjet device 40 connected to a positive pressure air source P. Preferably,the positive pressure air jet from the air jet device 40 levitates thetop several sheets in the supported sheet stack 50, while the vacuum atthe plenum 32 is effective through its ports to cause the topmostlevitated sheet from the stack 50 to thereafter be acquired at theplenum 32 for separation from the sheet stack 50. Additional positivepressure air jets from the air jet device 40 helps to assure separationof subsequent sheets from the acquired topmost sheet. To further assureseparation of sheets from the sheet stack, the lifting mechanism (forexample, L in FIG. 1) preferably presents the top sheet a specifieddistance from the vacuum plenum 32.

FIG. 3 is a side elevational view of a cross-section of the exemplaryreceiver sheet supply and feeding apparatus 10 taken along lines 3-3 ofFIG. 2, particularly showing the platform 14 lifting mechanism. Each ofthe lifting chains have a link 22 extending through respective slots 12a (FIG. 1) in the front and rear upstanding walls of the hopper 12. Thelinks 22 are connected to a shaft 24 a supported in brackets 24 bextending from the underside of the platform 14. Tension cables 26 arerespectively connected, at the ends 26 a, 26 b thereof, to the front andrear upstanding wall of the hopper 12. The cables 26 are respectivelythreaded over their associated first pulleys 24 and under second pulleys28 mounted on a shaft 28 a supported in the brackets 28 b extending fromthe underside of the platform 14.

In FIG. 3, the sheet stack supporting platform 14 is shown in its mostelevated position in solid lines, and in its lowest position in phantom.During the operation of the lifting mechanism L, an appropriate signalto the motor M·sub·1 causes the motor to rotate the gear set 16 (FIG.1), such as either clockwise to lower the platform 14 toward the lowestposition or counterclockwise to raise the platform toward its mostelevated position. Rotation of the gear set 16 moves the lifting chains20 (FIG. 1) in their closed loop paths, thereby imparting verticalmovement to the links 22. This movement, in turn, moves the shaft 24 a,and thus the platform 14, and as well as its brackets 24 b and firstpulleys 24. The platform 14 is maintained substantially level in itsmovement by the action of the tension cables 26, which cooperativelymove the second pulleys 28, and thus, the shaft 28 a and the brackets 28b of the platform 14.

FIG. 4 is an end view, on an enlarged scale and with portions removed,of a portion of the receiver sheet supply and feeding apparatus 10,particularly showing the feed head assembly 30 thereof, taken along thelines 4-4 of FIG. 3. Preferably, maintaining the topmost sheet 51 at thepredetermined level is accomplished by one or more sheet detectingswitches 80, which controls the operation of the motor M·sub·1 foractuating the lifting mechanism L, (more described below), to raise theplatform 14 through a predetermined increment. On the other hand,lowering of the platform 14 is usually accomplished by some externallyproduced signal to the motor which tells the motor to rotate until theplatform 14 reaches the down switch 21 that signals the motor M·sub·1 tostop, often bringing the platform 14 to its lowest position.

Of course, other precisely controllable lifting mechanisms, such as wormgears, lead screws, or scissors linkages are suitable for use in theelevation control for the sheet stack supporting platform 14 accordingto these embodiments, and other suitable mechanisms without limitation.

Preferably, the lower surface 32 a of the plenum 32 of the sheet feedhead assembly 30 has a particularly configured shape, so as to providefor a specific corrugation of an acquired sheet 51. As the top sheets 51in the supported sheet stack 50 are levitated, the topmost sheet 51preferably contacts the outer winged portions 32 b of the surface 32 a.A minimal pressure is exerted on the sheet 51 to help in forming acontrolled corrugation to the sheet 51. This establishes a consistentspacing for the center portion of the sheet 51 from the center portionof the plenum 32. As such, the access time for a sheet 51 to be acquiredat the plenum 32 is often repeatably consistent and readily predictable.

The interactions of the plenum 32 and the air jet device 40 attempt toassure that control over the sheet 51, as it is acquired at the plenum32, is not lost. Further, corrugation of the sheet 51 contorts the sheet51 in an unnatural manner. Since subsequent sheets 51 are not subjectedto the same forces, at the same time, as is the topmost sheet 51, suchsubsequent sheets 51 are unable to contort in the same manner.Accordingly, the subsequent sheets 51 are effectively separated from thetopmost sheet 51 as it is being acquired at the plenum 32.

As noted above, it is important for proper operation of the sheet supplyand feeding apparatus 10, according to this embodiment, for the level ofthe topmost sheet 51 in the stack 50 supported on the platform 14 to bemaintained at a predetermined height relative to the plenum 32. Thelevel is selected to be in a range where the topmost sheet 51, whenlevitated by the first air jet arrangement 42, is close enough to theplenum 32 to be readily acquired by the vacuum forces from the plenum32, within a repeatable time frame, but yet far enough away from theplenum 32 to assure that the sheet being acquired is not pinned againstthe plenum 32.

Preferably, each of the switches 80, as noted above, are designed todetect the level of the topmost sheet 51. Such switches 80, as known inthe art, could be for example, a paper guide that rides against thesheet 51 with very little downward pressure, at the highest level ofacceptable corrugation, as found in U.S. Pat. No. 5,823,527, in the nameof Burlew et al. Additionally, paper level actuators could be integratedinto an optical switch so as to cause limited pressure on the sheet 51.The switches 80 can be read during the feed interval, and if necessary,will transmit a signal to the lifting mechanism L to raise the platform14 in one or more increments. Preferably the increments can maintain theproper sheet level. The location of the switches 80 at the highest levelof acceptable corrugation is an advantage in that each of the switches80 can sense the location of sheets 51 which may be severely curled andstill not pin the sheet 51 to the plenum 32.

Referring back to FIG. 1, to further assure separation of sheets fromthe sheet stack and the switching over to another stack, the liftingmechanism L can present the top sheet a desirable distance from thevacuum plenum, in response to a second signal that originates from asecondary source other than the switches 80, such as by a microprocessor90 executing source code, or hardware logic.

FIG. 5 is a scheme illustrating an exemplary reproduction device 500with two feeding apparatuses 502, 504 similar as described above withFIGS. 1-4. In each of feeding apparatus 502, 504 there is a platform506, 508 supporting stack 510, 512. The platform 506, 508 is coupledwith an elevating stepper motor 514, 516. Sheets 518, 520 in a stack510, 512 are separated and transported by a feed head assembly 522, 524.The stack height is measured with level sensors 526, 528. An additionalpaper out sensor 527, 529 gives a signal if no sheet 518, 520 isremaining on the platform 506, 508. A reference position of the platform506, 508 is detected with down switches 530, 532. To count the number ofseparated and transported sheets 518, 520 an optical edge sensor 534,536 is arranged in the transport path 538, 540. The sheets 518, 520 aretransported to a printing unit 542. After printing the sheets 518, 520are discarded in a piling apparatus 544. The piling apparatus contains aplatform 546 to discard the sheets 518, 520 in a stack 548. The stack548 is lowered with the help of a stepper motor 550 whereby the bottomposition is detected with a down switch 552.

As shown in FIG. 5 all active and sensor elements are connected to acontrol system 554 for the reproduction device 500. To input, processand display data the control system 554 is connected to a computersystem 556 with a keyboard 558 and a monitor 560. Preferably, softwarefor controlling feeding, of types known in the art, is modified inaccordance with the present invention to provide the functionalitydescribed herein.

With FIGS. 6-9 it will be described below how the stack-advancing may beperformed according to various further aspects of the invention.Referring now to FIG. 6 (with reference to FIG. 5), a first procedure ispresented wherein a number of steps needed to advance the stacks 510,512 from a bottom most to a top most position is determined. Thisprocedure is preferably done when the printing unit 500 is manufacturedand the feeding apparatuses 502, 504 are mounted, or by field service ifthey have to be changed or repaired. After starting the procedure bycalling up a program in the computer system 556, first a total possibledisplacement count is initialized to a nominal value N·sub·T. TheInitialized value N·sub·T is stored in Non-Volatile Memory (“NVM”, forexample battery-backed memory, flash memory, etc.), also referred toherein as “persistent memory”, within the control system 554. Next acomplete stack 510, 512 is advanced stepwise with the stepper motor 514,516 while sheets 518, 520 are separated with the head assembly 522, 524.This is performed with the control system 554. Just before every feedthe current step count N·sub·T,C of the motor 514, 516 is recorded. Asuccessful feed is verified with a signal from the edge sensor 534, 536.This procedure goes on until the paper out sensor 527, 529 generates apaper out signal. If so, the current step count N·sub·T,C, which is thetotal number of steps needed to feed a stack of sheets starting from theinitial lowest position of platform 506, 508, is saved as the new totalpossible displacement count N·sub·T in the NVM memory, therebyoverwriting the nominal initialized value N·sub·T.

In FIG. 6 there is shown a platform 506, 508 in a bottom-most position(solid lines) and a top-most position (dashed lines). The just-describedprocedure starts at the bottom-most position where the platform 506, 508closes the down switch 530, 532. This responds to the reference positionwith the step count zero. In vertical direction the step count is shown.After feeding all sheets 510, 512 the empty platform 506, 508 wouldactivate the level sensor 526, 528 in the top-most position. In thisposition the step count reaches N·sub·T.

The new total possible displacement count N·sub·T may be checked todetermine whether it lies in a predetermined range of values. If not anerror message may be displayed on the monitor 560. In this case aservice person could do further checking.

Referring now to FIGS. 5 and 6, the number of steps needed for thestepper motor 514 (FIG. 5) to advance the stack 510 for feeding K sheetsmay be determined, wherein K is the number of sheets 518 that shouldremain in the stack 510 before the scheduling of future feeding goes tothe other stack 512 in a continuous mode. For example, K may be themaximum number of sheets that can potentially be scheduled in advance.This paper low displacement procedure is automatically realized byrecording the number of steps N·sub·K required to feed K sheets at somepoint during the reproduction process before only K sheets are left inthe stack 510, 512.

A paper-low value, N·sub·L, may be determined by subtracting N·sub·Kfrom N·sub·T. N·sub·L may be used to signal a user that paper is almostout in a particular hopper, or it may be used to initiate transfer toanother paper supply when paper is feeding in continuous mode.Preferably, K corresponds to a number of sheet feeds already fed from acorresponding supply before N·sub·L is reached. This value N·sub·L isalso stored in the memory, preferably volatile Random Access Memory(RAM) rather than NVM.

The system may be initialized with a value N·sub·L that represents anominal paper-low value. For example, if it is determined that an accessto the hopper 12 of apparatus 502 or 504 or a paper attributes changeoccurred, a paper-low displacement count may be initialized to a nominallow paper value N·sub·L. N·sub·L may be chosen to either correspond to athickest possible paper to ensure that paper will never run out in adrawer or N·sub·L may be chosen to correspond to a thinnest possiblepaper to ensure that excess paper is not left in a drawer.

With the motor 514 the stack is advanced up to the level of thefeed-head assembly 522, as shown in FIG. 7. The arrival at the feed-headassembly is confirmed by the level sensor 526. After the level sensor526 is activated the current step count is recorded as N·sub·0 in thememory. K sheets are fed, and the corresponding step count N·sub·1 isrecorded. The number of step counts corresponding to K sheets isN·sub·K=N·sub·1−N·sub·0. Finally a new paper low nominal value N·sub·Lmay be calculated as the difference between the total possibledisplacement N·sub·T and N·sub·K, N·sub·L=N·sub·T−N·sub·K. The stack 510has now the position shown in FIG. 8.

After determining the paper low value, N·sub·L, feeding may continueuntil the actual step count reaches N·sub·L. The platform 506 has thenthe level shown in FIG. 9. The scheduling from stack 510 will be stoppedand is continued with feeding apparatus 504 activated with the controlsystem 554. The feeding out of apparatus 504 is done in the same way asdescribed with feeding apparatus 502.

While the switching over from one feeding apparatus 502 to the nextfeeding apparatus 504 has been described with the remaining sheet numberK, it should be clear that the switching over could be delayed byfeeding J additional sheets with the feeding apparatus 502. For example,after paper low N·sub·L is reached, allow scheduling of J additionalfeeds in a manner to insure that not more than K feeds occur from thatpoint prior to switching the supplies. I.e., if six additional feeds (J)are scheduled when paper low N·sub·L is reached, allow K−6 (k−J) morefeeds to be scheduled prior to switching to feeding apparatus 504.

The present embodiments described herein, provide the ability to moreeffectively and reliably control stack-advancing in a reproductiondevice, by automatically calibrating the counts for the stepper motorsM1, 514, 516. Although described in the setting of a reproduction deviceutilizing a top feed vacuum feeder 502, 504 and switches 80, 526, 528that generate a signal to indicate an increment, it should be understoodthat the present embodiments could be implemented in a reproductiondevice that utilizes other types of feeders and switches, or in anoff-line configuration (a paper supply not connected to a reproductiondevice), or with a post-fuser inserter.

The disclosed method provides a number of advantages and applications.Utilizing the disclosed embodiments, the present invention allows bettercontrol over the number of sheets remaining during a continuous modeswap even if the sheet attributes and the mechanical tolerances changeor vary from stack to stack.

It should also be understood that the programs, processes, methods andsystems described herein are not related or limited to any particulartype of hardware, such as TTL logic or computer software, or both.Various types of general purpose or specialized processors, such asmicrocontrollers may be used with or perform operations in accordancewith the teachings described herein.

In view of the wide variety of embodiments to which the principles ofthe present invention can be applied, it should be understood that theillustrated embodiments are exemplary only, and should not be taken aslimiting the scope of the present invention. For example, more or fewerelements may be used in the drawings and signals may include analog,digital, or both. While various elements of the preferred embodimentshave been described as being implemented in hardware, in otherembodiments in software implementations may alternatively be used, andvice-versa. For example, the said stepper motor, could be any type ofmotor with feedback for platform movement such as an encoder or apotentiometer.

Although the invention has been described and illustrated with referenceto specific illustrative embodiments thereof, it is not intended thatthe invention be limited to those illustrative embodiments. Thoseskilled in the art will recognize that variations and modifications canbe made without departing from the true scope and spirit of theinvention as defined by the claims that follow. It is therefore intendedto include within the invention all such variations and modifications asfall within the scope of the appended claims and equivalents thereof.

1. A method for controlling sheet stack advancing, comprising:determining a sheet thickness by measuring a displacement of a platformcorresponding to a known number of sheet feeds by said feedhead, saidsheets resting upon said platform; determining a distance of saidplatform relative to a feedhead corresponding to a predetermined numberof sheets having said sheet thickness to be left in a sheet supply;switching scheduling of future feeds to another sheet supply when saidplatform is said distance from said feedhead.
 2. The method of claim 1,further comprising driving said platform with a stepper motor, andexpressing said distance as stepper motor counts.
 3. The method of claim1, further comprising determining said distance prior to said platformbeing at said distance relative to said feedhead.
 4. The method of claim1, further comprising storing said distance in memory.
 5. A method forcontrolling sheet stack advancing, comprising: determining a maximumtravel of a platform and storing it in a memory, said sheet stackresting upon said platform; advancing said platform with a motor from abottom-most to a top-most height position and performing sheetseparating and feeding; determining a current platform travel beforeevery feed; saving said current platform travel in said memory andcomparing said current platform travel with a nominal platform travel,and updating said maximum travel in memory each time said platform iscompletely emptied of sheets.
 6. The method of 5, further comprisinggenerating an error signal if a difference between said current platformtravel and said nominal platform travel is greater than a predeterminedvalue.
 7. A method for controlling stack advancing in a reproductionapparatus, comprising: determining maximum platform displacement,N·sub·T, and storing it in memory, a stack of sheets resting on saidplatform; advancing said platform and performing sheet separating andfeeding for K sheets; recording a current platform displacement,N·sub·K, that occurred during feeding said K sheets; and, calculating apaper low displacement N·sub·L=N·sub·T−N·sub·K and storing N·sub·L inmemory.
 8. The method of claim 7, further comprising initializingN·sub·L to a nominal value and storing it in memory.
 9. The method ofclaim 7, wherein reaching N·sub·L initiates switching over to feed fromanother stack loaded with the same sheet attributes.
 10. The method ofclaim 7, comprising initializing N·sub·L if a renewal of the stackoccurs.
 11. The method of claim 7, further comprising initializingN·sub·L in response to a change of sheet attributes.
 12. The method ofclaim 7, further comprising initializing N·sub·L to a nominal value,storing it in memory, and replacing it with a determined N·sub·L forthat stack.
 13. The method of claim 7, further comprising driving saidplatform with a stepper motor, and expressing said displacement asstepper motor counts.
 14. A method for controlling stack-advancing in areproduction apparatus, comprising: driving a platform in steps with alifting motor and performing sheet separating and feeding; initializinga paper-low displacement, N·sub·L, of said platform to a nominal numberof said steps and storing it in memory; determining a number of steps ofsaid lifting motor to achieve movement from a bottom position to a topposition of said platform, N·sub·T, and storing it in a memory;separating and feeding K sheets and recording in memory an actual numberof said steps corresponding to feeding said K sheets, N·sub·K; replacingsaid nominal number of steps with N·sub·T−N·sub·K in memory.
 15. Themethod of claim 14, wherein reaching N·sub·L initiates switching over tofeed from another stack loaded with the same sheet attributes.
 16. Themethod of claim 14, comprising initializing N·sub·L if a renewal of thestack occurs.
 17. The method of claim 14, further comprisinginitializing N·sub·L in response to a change of sheet attributes. 18.The method of claim 14, further comprising initializing N·sub·L to anominal value, storing it in memory, and replacing it with a determinedN·sub·L for that stack.
 19. A method for controlling sheet stackadvancing, comprising: a) determining stack position corresponding to apredetermined number of sheets remaining in a sheet supply to indicateda paper low nominal valve; and b) controlling sheet feeding in responsethereto.